1. Field of the Invention
This invention is generally related to the field of magnetic resonance imaging (MRI) utilizing nuclear magnetic resonance (NMR) phenomena. It is more particularly directed to method and apparatus for efficiently and safely planning and performing interventional medical procedures using MR imaging of interventional device(s) and of relevant patient anatomy.
2. Related Applications
This application is related to commonly assigned copending U.S. application Ser. No. 07/650,215 filed Feb. 4, 1991 entitled "Real Time MR Imaging Inside Gantry Room" and naming Kaufman, Arakara, Kramer, McCarten and Hawryszko as inventors. The entire content of such prior related application is hereby incorporated by reference.
3. Description of Related Art
Modern MRI systems now include some with magnet structures that provide rather free and open lateral access to the patient image volume during MRI procedures. For example, the ACCESS.RTM. low field MRI system available from Toshiba Corporation is such a system. Such open magnet architecture and MRI-compatible interventional instruments have already been recognized as greatly facilitating the use of MRI in interventional procedures. See, for example, Dietrich et al, "Low-Field Design Eases MRI-Guided Biopsies," Diagnostic Imagino, pages 139-143, March 1991. Real time fluoroscopy MRI is now also known and is expected to soon become commercially available. The above-identified related copending application describes such real time MRI fluoroscopy with provisions for including the MR image display inside the gantry room thus further facilitating use of MRI in interventional procedures.
Interventional procedures (e.g., biopsy, treatment, stereotaxy) require placement of various devices in or around the patient body. For example, even the introduction of a simple needle deeply within the human anatomy requires the attending physician to carefully plan the needle trajectory (e.g., angle, depth of penetration, etc.) so as to reach the desired end point with a minimum of damage to intervening tissues. In short, it is important that the interventional devices properly avoid certain parts of the patient anatomy while it is just as important that the device (or at least the active portion of the device) accurately reach, penetrate, etc. other particular portions of the patient anatomy. Even when real time fluoroscopic MRI is used as an aid in this process, the process requires apriori decision making and typically results in relatively slow procedure execution.
Of course it has long been known to use prior images of relevant patient anatomy (e.g., X-ray, CT Scans, MRI, etc) for the attending physician to study in advance to plan an optimum trajectory for the interventional device. However, inherent inaccuracies in the MRI process make such planning quite difficult if not impossible using MRI. Furthermore, due to common mis-perceptions in the medical community about absolute accuracy of MRI, such planning can even be deceptive.
In particular, although MRI provides very good relative information about tissue structure and location, it inherently can never provide absolute dimensional accuracy for interventional procedures planned on the basis of mere inspection of prior MR images of patient anatomy alone. In effect, the dimensional scale of an MR image changes from point-to-point throughout the image in a complex and somewhat unpredictable way. Thus, merely measuring the distance between two points on the two-dimensional image surface does not mean that the measured distance will correspond to the actual distance between those two indicated points of the anatomy within the human body. It can perhaps be analogized to the drawing of an image on a rubber sheet which is then deflected so as to be non-planar. A two-dimensional projection of such a distorted rubber sheet image does not provide a constant scale image on which accurate absolute dimensions can be measured.
Even though absolute dimensional accuracy is not possible using MR images, the relative location of objects within an MR image can be highly accurate. Thus, if the image shows two structures intersecting, it can be safely assumed that they actually do intersect. Similarly, if they do not intersect in the image, then it can be safely assumed that they do not intersect.